This invention relates to a vacuum power servo booster, and more particularly, to a technique for coupling together shells of the vacuum power servo booster, which shells fixedly secure a peripheral end portion of a diaphragm.
Conventional vacuum power servo boosters such as a power brake servo booster and a power clutch servo booster include a pair of shells forming an outer casing and a flexible diaphragm supported between the shells which divides the casing into two chambers. FIG. 1 shows such a conventional power brake servo booster 21 in which the diaphragm is secured by the shells as shown. At peripheral end portions of a pair of shells 22, 23, flanges 25, 26 extend radially outwardly in an opposing relationship to couple the shells 22, 23 together. The peripheral end portion of a diaphragm 24 is interposed between the opposing flanges. This type of power servo booster is bulky in that the presence of flanges 25, 26 prevents the booster 21 from being made more compact relative to the effective diameter of the shells, the effective diameter being determinative of the power output of the booster.
In order to overcome this drawback, there has been proposed a structure as described in U.S. Pat. No. 3,656,413 in which, as shown in FIGS. 2 and 4 thereof, a diaphragm 5 is provided with beading 51 around its outer peripheral edge. During assembly of the servo-device, the beaded edge 51 is clamped between a cylindrical wall 11, a channel 22 formed at the outer periphery of a cover plate 2 and a flanged ring 6 which is secured to the inner face of the cover plate 2. The channel 22 has spaced apertures and deformable lugs 12 on the adjacent end portions of the cylinder wall 11 which pass through the apertures and are bent through about 90.degree. to secure the cover plate 2 to the cylinder 1. It is intended with this structure to minimize the difference between the effective diameter of the cylinder and the external diameter of the power servo device to thus obtain high output in a compact device.
However, during assembly, the outer peripheral surface of the beading 21 may be damaged by the inner peripheral surface of the cylinder wall 11, particularly by the inner end corner of the cylinder wall due to sliding contact therebetween. Further, after the cover plate 2 has been secured to the cylinder 1, it is almost impossible to subsequently disassemble the servo device.